There are 2 "queens"/crowns in expansion with "sell this card for 4 florins "ability.
Is it legal to use crown/queen and then sell only the crown with king remaining?
Thematically, selling a queen is equivalent of expending your influence in the royal pair, so I think you should not be able to sell just the queen. The Living Rules add a sentence saying that you must sell a coronated pair as a set, (a set for 6 florins in the expansion cases you mention):
E3. Sell card.
Discard a card and receive 2 Florins from China. The card sold must come from your Hand or your Tableau. If the latter, Discard all
Queens, Vassals, and Tokens on the sold card. A coronated pair (I1) must be sold as a set, for 4 Florins total. See “Discard” in the glossary.